I haven't used a vacation feeder in a long time, so I cant remember if that happened to me or not. I would say it is from feeder, since it wasn't there before. Its good to know that I should expect the same if I use a feeder.
I think when feeders dissolve or break apart, the particles could float up and stay there to break into more fine particles.
Does it wipe off easy with a magnet cleaner?